Add unique machine identifier values to gitlab-runner

Scope

We should ensure that a gitlab-runner instance is associated with a new unique ID. This unique ID should live in a separate config TOML file so as to discourage having it being copied around with the config.toml. The system ID should be generated (if missing) whenever the configuration is loaded. Whenever we end up generating a new unique ID value, we should log that fact with the short token with INFO level.

When loadConfig() is executed, we'll want to run the check in case we need to save back the file with any missing unique_id values.

Background

• Related discussion here

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